Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described in terms of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart.
Present day cardiac pacemakers are typically designed to be implanted in a "pocket" of fatty tissue near the patient's upper breast or lower abdomen. Accordingly, the electronic circuits within the pacemaker are hermetically sealed within a housing made of a material compatible with body tissue. Electrical connection is made with the pacemaker electronic circuits via feedthrough terminals that pass through the hermetically sealed housing. The feedthrough terminals are electrically connected to a connector receptacle in the pacemaker housing for receiving a connector pin at the proximal end of a pacing lead. The lead has a distal end having electrodes attached to the desired tissue location. For cardiac pacing, such a lead is typically inserted through one of the main veins of the patient, for example, the superior vena cava so that the distal end of the lead is directed inside the heart.
Good electrical contact must be maintained between the connector pin at the proximal end of the pacing lead and the pacing lead receptacle on the pacemaker. Further, the connection must be secure so that it does not come apart during use yet it must be detachable in the event the pacemaker or lead needs to be replaced. Moreover, the connection must at all times remain insulated and sealed from body fluids; such fluids are conductive and could cause an electrical short if permitted to infiltrate the connector assembly.
After pacing lead fixation, and before the pacemaker is connected, a final check is made with X-ray to insure that the entire intravascular part of the lead is correctly positioned. The connector pin at the proximal end of the pacing lead is then connected to the pacemaker by inserting it in the connector pin receptacle and pushing the connector pin all the way into the receptacle. Use of the correct connector pin (or connector pin adapter) assures that the connector pin makes a tight fit in the receptacle.
The connector pin is secured within the connector pin receptacle and good electrical contact between the pin and receptacle is assured by means of a setscrew received by a threaded opening in the pacemaker housing. The tightened setscrew is recessed within the threaded opening and is isolated electrically and from body fluids by an insulative (for example, rubber) sealing plug which, in accordance with one example of the prior art, is inserted in the portion of the threaded opening over the setscrew. In accordance with another example of the prior art, the setscrew is covered by an insulative plug comprising a self-sealing septum having a normally sealed passage in the center for receiving a hex key or wrench for turning the setscrew. Insertion of the hex key into the septum passage pushes the septum rubber into the hex cavity of the setscrew. Besides preventing full insertion of the hex key, the rubber tends to become pinched between the hex key and the sharp edges of the hex cavity resulting in shearing of the rubber and accumulation of rubber particles in the screw hex cavity. Further, the damage to the septum passage can destroy the integrity of the seal resulting in leakage of bodily fluids into the threaded setscrew opening which can cause electrical malfunctioning of the pacemaker. Further, damage to the septum passage can result in rubber particles entering the bodily cavity or pocket in which the implantable medical device is carried.
Accordingly, it is an overall object of the present invention to provide a septum-type insulating plug which overcomes the aforesaid problems.
Specifically, it is an object of the present invention to provide a septum-type insulation plug constructed so as to allow full insertion of the hex key while preventing damage to the plug and resulting leakage and dislodgement of rubber particles from the septum.